In some prior trolley bus heads, the top of the cradle includes a wiper-carrier part having a groove with inclined faces. A replaceable wiper is received by conical wedging in the groove of said wiper-carrier part and a safety plate is provided to prevent the replaceable wiper being extracted during reversing, given that the conical wedging effect is naturally disposed to tend to increase clamping while the trolley bus is moving forwards.
The wiper is worn away and must be changed very often, often as much as once a day. Thus, although such a sliding and wedging mounting is mechanically safe, it suffers from being difficult to disassemble: the operator must begin by unscrewing the safety plate and then take a tool for applying a shock to extract the replaceable wiper, and then re-assemble in reverse order using a new wiper.
Further, the replaceable wiper is relatively expensive to manufacture by virtue of its inclined slope surfaces: it must be machined from a part cut out from a plate, and the machining must be to within tight tolerances in order to ensure that adequate electric contact is made with the corresponding faces in the wiper-receiving groove in the wiper-carrier. Published French patent application No. 2,320,204 gives an illustration of a wiper of this type for use with a head which comprises a wiper mounted at each end of a rocker arm.
However, electrical contact is not as good as it should be because of friction between the smooth surfaces of the replaceable wiper and of the wiper-carrier, with the smooth nature of these surface being essential by virtue of the wiper being received in its carrier by sliding: it turns out that contact is limited to localized zones or points, which not only have an effect on the quality of the contact for current transmission purposes, but also runs the danger of giving rise to unwanted heating by virtue of the current being concentrated in just a few zones.
Other prior trolley bus heads use a replaceable wiper which is not received by sliding, and make use of a two-part cradle with the parts being bolted together.
Swiss patent No. 359,463 describes a trolley bus head including a top portion which is bolted on either side of a longitudinal mid plane, thereby holding a replaceable wiper against a smooth support and contact face.
In practice, bolting is not a satisfactory way of holding a wiper. Firstly the torque exerted on the bolts must be accurately applied since too little torque leaves a loose assembly with consequent poor electrical contact while too much torque runs the risk of cracking the replaceable wiper. Secondly, the machining tolerances both for the wiper and for the corresponding portions of the cradle are necessarily tight. Finally, when replacing a wiper, the head cannot be disassembled rapidly since the bolts must first be undone and then they must be re-tightened accurately.
German patent No. 974,463 describes a technique similar to the above technique but in which the bolting takes place in a longitudinal mid plane. The quality of the electric contact is improved relative to the head described in the above-specified Swiss patent by virtue of two flat contact blades sandwiched between the replaceable wiper and a support face of the cradle, however the above-mentioned drawbacks concerning rigidly maintaining a replaceable wiper by bolting remain.
Generally speaking, with the trolley bus heads described in the above-specified German and Swiss patents, it should be observed that the contact obtained between the replaceable wiper and the associated support face occurs, in point of fact, at a few points only. This makes it important to have a wiper whose bottom face is large: and given that the wiper must be also be sufficiently rigid to withstand the compression forces exerted thereon during clamping, the resulting wiper cannot avoid being bulky, and thus relatively expensive per wiper.
The state of the art may finally be illustrated by U.S. Pat. No. 2,508,531 which also describes the use of bolts for retaining a wiper. The cradle in the trolley bus head described has cooling side fins, which shows the extent to which there are overheating problems to be overcome. Preferred implementations of the present invention provide a trolley bus head which is more effective than those mentioned above.
More particularly, preferred implementations of the invention enable the head to be rapidly and easily dismounted without using special tools in order to replace a worn wiper.
In addition, preferred implementations of the invention make it possible to use a replaceable wiper whose shape is suitable for cheap mass production, in particular by using drawing means, while still providing good electrical contact.